The elastic scattering of heavy ions at large angles has received much attention since the measurements of Braun-Munzinger et at. 1 first revealed an enhanced cross section for scattering of 16 0+ 28 Si at center-of-mass angles larger than 100°. 2 The backward-angle enhancement seems to be a general feature for projectiles and targets in this mass region, although the size of the enhancement can vary markedly from system to system. A number of explanations have been offered for this phenomenon, such as Regge poles, 3 resonances, 4 parity-dependent potentials, 5 diffraction, 6 and particle exchange. 7 The regular broad structures appearing in the excitation functions could also be explained by orbiting in the mutual potential. 8 It would be desirable to have additional experimental data which might help in distinguishing among these various pictures. Since the interpretation of backward-angle enhancement in terms of the orbiting of projectile 6 M 0 Berlanger et al. 9 and target should have general consequences for inelastic scattering as well, 9 we have investigated the gross features of the inelastic scattering at backward angles. Experimental data of this type are reported here for the first time. We find both for the system 12 C + 20 Ne, on which we report here, and for other systems as well, 10 a strong enhancement of the cross section for inelastic scattering with large negative Q values at backward angles. The gross features we observe for the inelastic scattering find a natural interpretation in terms of orbiting and support this mechanism as the origin of backward-angle enhancement of elastic scattering as well.A natural carbon target was bombarded with beams of 50-to 80-MeV 20 Ne 4+ from the Oak Ridge isochronous cyclotron. The reaction products with Z> 3 were identified with a positionsensitive AE-E counter 11 which spanned an angular range of 9° in 1° steps. Angular distributions An experimental study of the 20 Ne + 12 C systems reveals a large cross section for inelastic scattering with large negative Q values at backward angles. The differential cross section is proportional to l/sin0 Coirio >100° and has characteristics consistent with the decay of an orbiting 20 Ne + 12 C dinuclear system.

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